Typical swather tractors include a frame which carries a cab and engine with a transverse frame assembly which carries on each of a pair of depending frame leg a respective one of a pair of driven wheels. The transverse frame assembly also carries a pair of lift arms each for supporting a header of the tractor to be moved in front of the tractor for crop harvesting.
At the rear of the frame is a transverse rear axle assembly which carries a pair of rear wheels. The rear wheels are castor wheels with a vertical castor shaft pivotal around a vertical axis at the respective end of the axle assembly and a single wheel mounted on a hub offset from the vertical axis so that the wheel hub defines a horizontal rotation axis for the wheel which is located at a position trailing the vertical castor axis.
The castor wheel is allowed to rotate freely and the steering for the tractor is provided by a differential in drive speed between the drive wheels so that the castor wheels merely follow the movement generated by the drive wheels and the frame.
Suspension for the rear wheels in some designs is provide merely by mounting the rear axle on a center horizontal pivot which allows the rear axle to pivot side so side as the ground level changes. Although in some more recent designs attempts have been made to improve suspension at the rear by splitting the rear axle into two parts each extending to a respective side and each pivotal about a respective pivot axis so that the outer ends of the parts can pivot upwardly and downwardly relative to the frame and independently of the other part. A gas bag suspension and shock absorber is provided between the axle part and the frame. The conventional castor wheel is mounted at the end of the axle part.
This arrangement of the castor wheel has been used for many years and is accepted as the standard arrangement for this type of tractor. One advantage of the system is that it makes the tractor very maneuverable and allows the tractor in effect to turn around a center point with the castor wheels turning at right angles to the center line of the tractor. In some cases also it allows the tractor to be reversed in direction for transport with the castor wheels turning through 180 degrees. Thus the castor wheels must be freely turnable to follow the direction of travel and must be free to rotate fully around the vertical castor axis. The terms “front” and “rear” used herein therefore refer for convenience to the direction of movement during harvesting.
Due to increases on operating speed both in the field and in transport, more attention is being given to improving the ride quality and to reduce loadings on the frame which can lead to structural damage if the frame is insufficiently robust.
Windrower (or swather) tractors are inherently quite rough to ride on. Cab suspension and rear walking beam suspension have been used to improve on ride but it would be desirable to also isolate the frame of the propulsion unit from shock so as to reduce stress in the frame members. The problem with a windrower is that it carries a variety of headers of different weights and sizes, making suspension tuning difficult.